The present invention relates to a servo circuit for rotary heads of a video tape recorder, or more particularly to a servo circuit of the rotary heads of the video tape recorder in which recording/reproduction is capable of being performed at two or more different tape speeds.
There is such a demand for a general video tape recorder for home use that recording/reproduction time should be switched between two or more different times by using a tape of the same length. In a video tape recorder meeting such a requirement, the tape feed rate is capable of being switched. Examples of the locus of the signal recorded on the tape by such a video tape recorder are illustratively shown in FIGS. 1 and 2. These are in connection with a two-head helical scan type of video tape recorder with FIG. 1 showing the condition of standard play, i.e. normal speed play. In the drawing, reference numeral 1 shows a head for channel 1, and numeral 3 a head for channel 2. The recording signals are recorded on the tape by alternately using the two heads 1 and 3. The arrows in FIGS. 1 and 2 show the running direction of the tape T, and the recording tracks 2a, 2b, 2c and so on are formed by the head 1, while the recording tracks 4a, 4b, 4c and so on are formed by the head 3. In order to increase the recording density, there is no guard band formed between the adjacent recording tracks and so adjacent tracks are arranged closely to each other. Reference character C shows a control track formed in the longitudinal direction of the tape, in which control pulses 9s are recorded in a predetermined relation with the recording tracks to enable the heads 1 and 3 to accurately trace the recording tracks 2a and 4a and so on at the time of reproduction.
The diagram of FIG. 2 shows a case of long play in which recording/reproduction is performed for twice the length of time and at one half the tape feed rate of FIG. 1 using the same tape. In the long play condition, tape feed speed is lower than in standard play and therefore the track recorded by the head 1 is partially rewritten by the head 3, thus forming a narrower track. In the case where the tape speed is one half of that in FIG. 1, just one half of each track is successively rewritten by the head 1 or 3 as shown in FIG. 2, thus forming recording tracks 2'a, 4'b, 2'b, 4'c and so on one half in width. This recording process is hereinafter called "the superimposed writting". Especially in the home video tape recorders now in use, no guard band is provided between the adjacent tracks in order to improve the recording density and instead, the gap angles or azimuth angle of the heads 1 and 3 with respect to the recording tracks are differentiated, thus preventing crosstalk from the adjacent tracks recorded by different heads, by use of the reproduction loss due to the difference of gap angles of the heads. For this reason, even when a recording track narrower than the heads is formed by the "superimposed writing" and the recorded data are read by a head wider than the track, crosstalk is reduced to a practically satisfactory degree.
A typical example of the rotation control system for the rotary head performing such a recording/reproduction is shown in FIG. 3, and a waveforms for explaining the operation thereof in FIG. 4. This control system is shown, for example, a book entitled "VTR" by Kenichi Sawazaki published by Corona Co., Ltd. Japan, pp. 141-143, 150-152. A synchronizing signal is separated from the video signal by a well known circuit not shown, thus providing a vertical synchronizing signal 11 as shown in FIG. 4a which is applied to a terminal 10. This vertical synchronizing signal is applied to a frequency divider 12 comprised of, for example, a monostable multivibrator, whereby it is divided by one half in frequency and produced as an output signal 13 shown in FIG. 4b. This output signal 13 is processed as required through a recording amplifier 14, applied to a control head 15, and recorded on the control track of the magnetic tape in the form of a control pulse 9 synchronized with the leading edge of the output signal 13 as shown in FIG. 4c.
The output signal 13 of the frequency divider 12 is also applied to one of the input terminals of a phase comparator circuit 16 of a rotary drum servo system 26. This drum servo system 26 so functions that the rotational phase of video heads 18a and 18b on a rotary drum 17 is synchronized with the phase of the output signal 13. In other words, the rotational phase of the rotary drum 17 is detected by a rotational phase detector means 191, and the detected signal is transformed into a rotational phase signal 20 as shown in FIG. 4d by a waveform shaping circuit 192, which signal is applied to the other input terminal of the phase comparator circuit 16. The output signal of the phase comparator circuit 16 is applied to a motor drive amplifier 21, so that the rotational phase of a motor 22 is controlled, thus driving the rotary drum 17 in synchronism with the vertical synchronizing signal. In this way, each field of video signal is recorded by being allotted exactly on each recording track as shown in FIG. 1 or FIG. 2.
In case of reproduction, the control pulse 9s is read from the control track of the magnetic tape by the control head 15, and the control pulse thus read is applied to the one input terminal of the phase comparator circuit 16 in place of the output signal 13 of the frequency divider 12 through a path not shown in the FIG. 3. The phase comparator circuit 16 compares the phase of the control pulse thus read with that of the rotational phase signal 20 produced in synchronism with the rotation of the rotary drum 17, thus controlling the rotary drum 17 to rotate in synchronism with the control pulse. As a result, the video heads 18a and 18b are capable of tracing accurately the same tracks as the recording tracks.
This operation is performed in the same manner for long play mode as for standard play mode, although the long play mode has a problem as mentioned below.
In the long play mode, a recording track one half the width of the heads is formed as previously described. In view of the fact that the heads perform the same tracing operation in reproduction as in recording, however, the head 1' for reproducing the track 2'x, for instance, traces the same position as the head 1 in recording operation. Thus the center line 5 of the head 1 is displaced from the center line 6 of the recording track 2'x as shown in the FIG. 2, with the result that the signal is read only by one end portion of the head 1'. If jitter occurs in the running speed of the magnetic tape, the head 1' may displace slightly to the right in FIG. 2 thus causing the head 1' in reproduction to fail to cover the entire width of the recording head 2'x, resulting in a mistracking. Especially in the case where reproduction is performed by another video tape recorder different from that used for recording and the head used for reproduction has a smaller width than the head used for recording, part of the recording track always fails to be traced. In such a case, even a slight jitter of the running speed of the tape causes the tracing of the reproduction head to be displaced completely from the recording track to be traced.
To prevent such a problem, the tracking by the reproduction head is required to be adjusted manually before reproduction, thus complicating the procedures for operation of the video tape recorder.